A conventional photomask comprises a patterned light-shielding film of opaque material, typically a metal such as chromium, on a transparent mask substrate, typically silica (quartz). In photomask manufacture, a photoresist is applied to the opaque-material side of a mask blank comprising a layer of the opaque material on a transparent mask substrate. The photoresist is patterned by an image-wise exposure and wet developed to produce a pattern of photoresist over the opaque layer. The mask blank containing the imaged photoresist is either wet or dry etched to remove the opaque material revealed by removal of the photoresist. When the photoresist is stripped after etching, a patterned layer of opaque material remains on the transparent substrate.
Typically, a positive-tone, novolac-type photoresist is exposed by a laser tool and developed with TMAH: a dilute solution of tetramethyl ammonium hydroxide. TMAH developer is a strong base. The exposed portions of the opaque layer are wet etched with a ceric salt, typically ceric ammonium nitrate in 10% nitric acid or a ceric salt in 10% perchloric acid. An acidic solution of a ceric salt is a strong oxidizing agent.
When a wet etch is used, it is common practice to perform both the develop step and wet etch step sequentially in the same process chamber of the process tool. These steps are typically carried out using a spray process in which the developer and wet etchant are sequentially sprayed onto a rotating mask substrate in a single process chamber. The usual sequence is: develop, rinse with deionized water, wet etch, rinse with deionized water, and dry.
The ceric ion is soluble in the acidic etch solution. Basic ceric salts precipitate in neutral solution or in the basic developer solution, however, and form small particles across the entire mask blank. Over time, a steady build up of yellow ceric hydroxide particles, orange ceric ammonium nitrate particles, and mixtures of these cerium-containing particles forms and deposits throughout the process chamber. The nozzles that spray the rinse water also become contaminated with cerium. The rinse water contains cerium in both soluble and insoluble forms.
If a dry etch is used in place of a wet etch, the mask blank containing the patterned photoresist is developed with TMAH, washed with deionized water, dried, and transferred to a reactive ion etch system for the dry etch of the chrome layer. Although the dry etch does not use a cerium etchant, the process tool used to develop the photoresist is typically contaminated with cerium from other processing.
Consequently, the mask blank is contaminated with cerium-containing particles during development. These particles are not removed by the deionized water rinse; rather, they produce chromium opaque defects on the resulting photomask due to micro-masking by the cerium-containing particles during dry etch of the chromium layer.
Most process tool manufacturers recommend a once-a-month manual washdown of the process tool with concentrated hydrochloric acid to remove the cerium compounds from the process tool. In addition to causing downtime, manual washdown is time consuming and inconvenient. Concentrated hydrochloric acid is corrosive and the precipitated cerium compounds and hydrochloric acid can react to produce chlorine gas and nitrogen dioxide.
Thus, a need exists for methods for removing precipitated particles of ceric salts from mask blanks prior to wet or dry etching to form photomasks and for methods for removing cerium deposits from process tools.